1. Field of the Invention
This invention relates to an optical information reading apparatus.
2. Description of the Related Art
In optical information reading apparatuses including an optical disk player or the like, reproduction of an optical disk which has pits recorded thereon at high density requires a reproduction optical system having high resolution corresponding to the recording density. For this purpose, To reproduce a reflective type of optical disk, most reproduction optical system shown in FIG. 1 comprises a collimator lens 3, a polarization beam splitter 4, a quarter wave plate 5, and an objective lens 6 on an optical path X between a semiconductor laser 2 and an optical disk 1 from the laser in turn. They causes a reading beam from the semiconductor laser 2 to be impinged on the optical disk 1. To receive a reflected beam from the optical disk 1, the reproducing optical system employs the objective lens 6 as a condenser lens, and a light receiver 7 is disposed on another optical path Y extending from the polarization beam splitter 4.
In the above case, OTF (Optical Transfer Function) of the reproducing optical system is an auto-correlation function of a pupil function of the objective lens 6. When the OTF is calculated on assumption that the objective lens 6 has a circular aperture, a cut-off spatial frequency fc can be express by 2NA/.lambda. where .lambda. represents the wavelength of the semiconductor laser 2 and NA represents the numerical aperture of the objective lens 6.
Stated another way, the reproducing optical system of FIG. 1 cannot read out information signals recorded on an optical disk at a high density whose spacial frequency is higher than the cut-off spatial frequency fc.
Recently, an optical disk having information signals recorded thereon at a high density is attempted to be reproduced by a conventional optical system. In such a conventional system, a plurality of pits may be included within a single beam spot 10 shown in FIG. 2(a), so that information signal corresponding to one pit cannot be read out from the optical disk. To solve this problem, there has been proposed an optical disk 1 with a reflective film 12 made of a material whose reflectance depends on the incident light intensity, so that the effective diameter of a beam spot received thereon is apparently reduced. For example, when the amplitude reflectance of the reflective film 12 increases substantially in proportion to the incident light intensity therein, the intensity distribution of the incident beam impinged onto the optical disk 1 shows an Airy pattern as shown in FIG. 2(b). Then a reflected beam from the optical disk 1 presents a lower amplitude reflectance in a peripheral portion so that the intensity of the peripheral portion becomes considerably lower than that of a central portion, as shown in FIG. 2(c). For this reason, the spot diameter of the reflected beam received by the reproducing optical system, i.e., effective spot diameter of the received beam is reduced. As a result, only one of the pits 11 in the beam spot 10 can be read. Thus, the proposed optical disk 1 described above enables conventional reproducing optical system to reproduce finely recorded information signals corresponding to the spatial frequency above the cut-off spatial frequency which is limited by the reproduction optical system.
However, when the reflective film 12 is formed of the material whose reflectance depends on the incident light intensity, the effective received beam spot diameter is reduced, while its scattering angle becomes larger. Specifically explaining, a beam diameter r emitted onto the optical disk 1 is defined by the following equation (1), EQU r=k.multidot.d.lambda./a (1)
wherein an aperture diameter of the objective lens 6 is a, the distance between the objective lens 6 and the optical disk 1 is d, and k is a proportional constant. When the beam diameter of the reflected beam becomes .alpha.r (.alpha.&lt;1) due to the intensity dependency of the reflectance of the film, an aperture diameter a' of the reflected beam distributing on the surface of the objective lens 6 is expressed by the following equation (2): EQU a'=k.multidot.d.lambda./.alpha.r (2)
Therefore, from the equations (1) and (2), the relationship between the aperture diameter a of the objective lens 6 and the aperture diameter a' of the reflected beam thereon is expressed by: EQU a'=a/.alpha. (3)
Thus, a'&lt;a stands. This means that if the aperture diameter of the objective lens 6 is equal to that of the condenser lens, reflected beams having large scattering angles will stray from the aperture of the objective lens and accordingly will not be led onto the objective lens.
The present invention has been made in view of the above-mentioned problems, and a main object thereof is to provide an optical information reading apparatus which is capable of efficiently receiving the reflected beam.
It is another object of the present invention to provide an optical information reading apparatus which is capable of efficiently receiving a reflected beam in case that the reflectance of a reflective film of an optical disk depends on an incident light intensity.